High energy is high frequency.

Energy of radio signal E = hf, Planck's constant times the frequency.

A magnetron is a high-power, high frequency vacuum tube.

The magnetron is the source of radio-frequency energy in RADAR systems

and in the microwave oven in your kitchen, for example.

highest

The energy of EM radiation depends on its frequency, and the lowest frequency corresponds to the lowest energy. Radio waves are at the low end of the energy and frequency spectrum.